Tried booking 71 PreFL MG5 LR Exclusive in with Islington Motors for Tesla Charging update.
Initially Islington suggested they expected it to be warranty work, but later contacted me to say that MG had issued a bulletin indicating that the software update was "considered to be an enhancement" and was therefore not covered by warranty, so they want to charge £165 for it (their standard diagnostic charge for out of warranty).
I've asked the service dept at Islington to provide me with a screenshot or photo of the bulletin from MG, they've said they're not sure what they're allowed to release but they're going to try and do something.
I'll end up paying the £165 I expect. I worked out that at 4mi/kWh, it'd take 2,200 miles of pure Tesla power (vs generic UK rapid charging at Zap-Map average rate) to recoup the cost - which I'm likely to cover over the lifetime of the vehicle. I also considered that it provides more charger availability, and may have a positive effect on resale value if the update has already been done.
ETA: Islington confirmed that for the 71 PreFL MG5 LR Exclusive, it is a software update only (no new or replacement hardware required), so anyone being quoted e.g. £800 because of additional hardware should perhaps ask to see the bulletin from MG?